The present invention relates to a catalytic composition which comprises a beta zeolite, a metal of group VIII, a metal of group VIB and optionally one or more oxides as carrier. The catalytic system of the present invention can be used for the hydrotreating of hydrocarbon mixtures and more specifically for the upgrading of hydrocarbon mixtures which boil within the naphtha range, containing sulfur impurities, i.e. in the hydrodesulfuration with contemporaneous skeleton isomerization and a reduced hydrogenation degree of the olefins contained in said hydrocarbons, the whole process being carried out in a single step. This catalytic system can be used, in particular, for the upgrading of mixtures of hydrocarbons which boil within the naphtha range deriving from cracking processes, preferably mixtures of hydrocarbons having a boiling point within the naphtha range deriving from FCC catalytic cracking (Fluid Catalytic Cracking).
Hydrocarbons which boil within the naphtha range deriving from FCC (i.e. gasoline cut) are used as blending component of gasolines. For this purpose, it is necessary for them to have a high octane number together with a low sulfur content, to conform with the law restrictions which are becoming more and more severe, in order to reduce the emission of contaminants. The sulfur present in gasoline mixtures in fact mainly comes ( greater than 90%) from the gasoline cut deriving from FCC.
This cut is also rich in olefins which have a high octane number. Hydrogenation processes used for desulfuration also hydrogenate the olefins present with a consequent considerable reduction in the octane number (RON and MON). The necessity has therefore been felt for finding a catalytic system which decreases the sulfur content in hydrocarbon mixtures which boil within the naphtha range and, at the same time, minimizes the octane loss (RON and MON), which can be achieved, for example, by the skeleton isomerization of the olefins present and/or by inhibiting the hydrogenation of the olefinic double bond.
The use of zeolites with a medium pore dimension as isomerization catalysts and the consequent recovery of octane in the charges already subjected to desulfuration are already known (U.S. Pat. No. 5,298,150, U.S. Pat. No. 5,320,742, U.S. Pat. No. 5,326,462, U.S. Pat. No. 5,318,690, U.S. Pat. No. 5,360,532, U.S. Pat. No. 5,500,108, U.S. Pat. No. 5,510,016, U.S. Pat. No. 5,554,274, U.S. Pat. No. 599,439). In these known processes, in order to obtain hydrodesulfuration with a reduced octane loss, it is necessary to operate in two steps, using in the first step catalysts suitable for desulfuration and in the second step catalysts for recovering the octane number.
U.S. Pat. No. 5,378,352 describes a process in a single step for desulfurating hydrocarbon fractions, with boiling points within the range of gasolines, using a catalyst which comprises a metal of group VIII, a metal of group VI, a zeolite selected from ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-35, ZSM-48, ZSM-50, MCM-22 and mordenite, and a metal oxide as ligand, with a process temperature preferably higher than 340xc2x0 C.
Some catalytic materials containing metals of group VIII and group VIB, a refractory carrier and a zeolite selected from ZSM-35, ZSM-5, mordenite and fujasite, are described in EP 442159, EP 437877, EP 434123 for the isomerization and disproportioning of olefins; in U.S. Pat. No. 4,343,692 for hydrodewaxing; in U.S. Pat. No. 4,519,900 for hydrodenitrogenation, in EP 072220 for a process in two steps comprising dewaxing and hydrodesulfuration; in U.S. Pat. No. 4,959,140 for a hydrocracking process in two steps.
We have now surprisingly found a new catalytic system which can be used for the hydrotreating of hydrocarbon mixtures and, more specifically, we have found a catalytic system with which it is possible to desulfurate, with high conversion values, mixtures of hydrocarbons that boil within the naphtha range containing sulfur and olefins and contemporaneously obtain the skeleton isomerization of the olefins present with a low hydrogenation degree of the olefinic double bond. This new catalytic system is also active at temperatures and pressures that are lower than those preferably used in the known art for desulfuration.
Both skeleton isomerization and reduced olefinic hydrogenation enable hydrocarbon mixtures to be obtained, which boil within the naphtha range with very low RON (research octane number) and MON (motor octane number) losses.
The catalytic compositions of the present invention can not only be used for the desulfuration of hydrocarbon cuts that boil within the xe2x80x9cheavy naphthaxe2x80x9d range (130xc2x0-250xc2x0 C.), i.e. cuts poor in olefins, but also feeds of xe2x80x9cfull range naphthaxe2x80x9d, which boil within the range of 35xc2x0-250xc2x0 C., i.e. in the case of cuts rich in olefins. In fact, the catalytic system of the present invention has a high selectivity for desulfuration with respect to hydrogenation, which represents an additional advantage in terms of octane recovery in the end-gasoline.
A first object of the present invention therefore relates to a catalytic composition which comprises a beta zeolite, a metal of group VIII, a metal of group VIB, and optionally one or more oxides as carrier.
Beta zeolite is a porous crystalline material described in U.S. Pat. No. 3,308,069, having a molar composition of oxides corresponding to the following formula:
[(x/n)M(1xc2x10.1xe2x88x92x)Q]AlO2*ySiO2*wH2O
wherein x is less than 1, preferably less than 0.75, y varies within the range of 5 to 100, w varies within the range of 0 to 4, M is a metal selected from metals of groups IA, IIA, IIIA, or is a transition metal, n is the valence of M and Q is a hydrogen ion, ammonium ion, an organic cation or a mixture of these. Preferably y is greater than 5 and less than 50.
According to a particularly preferred aspect of the present invention the beta zeolite is in acid form i.e. in the form in which the cationic sites of the zeolite are prevalently occupied by hydrogen ions. It is especially preferable for at least 80% of the cationic sites to be occupied by hydrogen ions.
According to an aspect of the present invention, when the catalytic composition comprises beta zeolite and metals of group VIII and group VIB, said zeolite is preferably present in a quantity ranging from 70 to 90%; when the catalytic composition also comprises one or more oxides as carrier, said zeolite is preferably present in a quantity ranging from 5 to 30% by weight with respect to the total weight of the catalyst.
The catalysts used in the present invention preferably contain Cobalt or Nickel as metal of group VIII, whereas the metal of group VIB is preferably selected from molybdenum or tungsten. According to a particularly preferred aspect, Co and Mo are used. The weight percentage of the metal of group VIII preferably varies from 1 to 10% with respect to the total weight of the catalyst, even more preferably from 2 to 6%; the weight percentage of the metal of group VIB preferably varies from 4 to 20% with respect to the total weight of the catalyst, even more preferably from 7 to 13%. The weight percentages of the metal of group VIB and the metal of group VIII refer to the content of metals expressed as metal element of group VIB and metal element of group VIII; in the end-catalyst the metals of group VIB and VIII are in the form of oxides. According to a particularly preferred aspect, the molar ratio between the metal of Group VIII and the metal of group VIB is less than or equal to 2, preferably less than or equal to 1.
The oxide used as carrier is preferably the oxide of an element Z selected from silicon, aluminum, titanium, zirconium and mixtures of these. The carrier of the catalytic composition can consist of one or more oxides and the oxide used is preferably alumina or alumina mixed with an oxide selected from silica and zirconia.
The catalytic compositions of the present invention can be prepared with traditional methods, for example by impregnation of the beta zeolite with a solution containing a salt of a metal of group VIB and a salt of a metal of group VIII, drying and calcination. The impregnation can also be effected using a solution containing a salt of a metal of group VIB and a solution containing a salt of a metal of group VIII.
When the catalyst contains one or more oxides as carrier it can be prepared by mixing the zeolite with the oxide, followed by extrusion, calcination, an optional exchange process which reduces the sodium content, drying, impregnation with a solution containing a salt of a metal of group VIB, drying, calcination and impregnation with a solution of a salt of a metal of group VIII, drying and calcination.
According to a particularly preferred aspect of the present invention, the catalytic compositions which contain one or more oxides as carrier are prepared by means of the sol-gel technique as follows:
a) an alcoholic dispersion is prepared, containing a soluble salt of the metal of group VIII, beta zeolite and one or more organic compounds capable of generating the supporting oxide or oxides;
b) an aqueous solution is prepared containing a soluble salt of the metal of group VIB and, optionally, tetraalkylammonium hydroxide having the formula R4NOH;
c) the alcoholic dispersion and the aqueous dispersion are mixed and a gel is obtained;
d) aging of the gel at a temperature ranging from 10 to 40xc2x0 C.;
e) drying of the gel;
f) calcination of the gel.
The catalytic compositions thus obtained have a high surface area ( greater than 200 m2/g) and a high pore volume ( greater than 0.5 cm3/g) with a distribution within the mesoporosity range.
In step a) of this preparation, the metal salt of group VIII is, for example, a nitrate, a hydroxide, an acetate, an oxalate, and preferably a nitrate.
The organic compound capable of generating the supporting oxide or oxides, by means of hydrolysis and subsequent gelations and calcination, is preferably the corresponding alkoxide or alkoxides, in which the alkoxide substituents have the formula (Rxe2x80x2O)xe2x80x94 wherein Rxe2x80x2 is an alkyl containing from 2 to 6 carbon atoms. The alkoxide is preferably an element Z selected from silicon, aluminum, titanium, zirconium and their mixtures; in particular, when Z is aluminum, it is a trialkoxide having the formula (Rxe2x80x2O)3Al, wherein Rxe2x80x2 is preferably an isopropyl or a sec-butyl; when Z is silicon, it is a tetraalkoxide having the formula (Rxe2x80x2O)4Si wherein Rxe2x80x2 is preferably ethyl and, when Z is Zr, it is an alkoxide having the formula (Rxe2x80x2O)4Zr wherein Rxe2x80x2 is preferably isopropyl.
In step b) the soluble salt of the metal of group VIB can be an acetate, an oxalate or ammonium salts, and is preferably an ammonium salt. The tetraalkylammonium hydroxide has the formula R4NOH wherein R is an alkyl group containing from 2 to 7 carbon atoms. According to a preferred aspect the solution in step b) also contains formamide (Drying Control Chemical Agent) which favours the stabilization of the porous structure during the drying phase.
The quantities of the reagents are selected in relation to the composition of the end-catalyst.
In step c), according to the preferred sequence, the solution of step b) is added to the suspension of step a)
In step d) the gel obtained is maintained at a temperature ranging from 10 to 40xc2x0 C., for a time of 15-25 hours.
Step e) is carried out at a temperature ranging from 80 to 120xc2x0 C.
Step f) is carried out at a temperature ranging from 400 to 600xc2x0 C.
According to another aspect of the present invention, the catalytic system containing one or more oxides as carrier can be prepared as follows:
a) an alcoholic dispersion is prepared, containing beta zeolite and one or more organic compounds capable of generating the supporting oxide or oxides;
b) an aqueous solution is prepared containing tetraalkylammonium hydroxide having the formula R4NOH;
c) the alcoholic dispersion and the aqueous dispersion are mixed and a gel is obtained;
d) aging of the gel at a temperature ranging from 10 to 40xc2x0 C.;
e) drying of the gel;
f) calcination of the gel;
g) impregnation of the calcined product with a solution containing a salt of a metal of group VIB, drying, calcination and impregnation with a solution of a salt of a metal of group VIII, drying and calcination.
The quantities of the reagents are selected in relation to the composition of the end-catalyst. The reagents used are the same as the sol-gel synthesis.
According to another aspect of the present invention, the catalytic compositions containing the supporting oxide or oxides can be prepared as follows:
a) an alcoholic dispersion is prepared, containing a soluble salt of the metal of group VIII and one or more organic compounds capable of generating the supporting oxide or oxides;
b) an aqueous solution is prepared containing a soluble salt of the metal of group VIB and, optionally, tetraalkylammonium hydroxide having the formula R4NOH;
c) the alcoholic dispersion and the aqueous dispersion are mixed and a gel is obtained;
d) aging of the gel at a temperature ranging from 10 to 40xc2x0 C.;
e) drying of the gel;
f) mechanical mixing of the dried product with beta zeolite;
g) calcination.
The reagents used are the same as the sol-gel synthesis.
The quantities of the reagents are selected in relation to the composition of the end-catalyst. According to another aspect of the present invention, the catalytic compositions containing one or more oxides as carrier can be prepared as follows:
a) impregnation of the carrier, consisting of one or more oxides, with a salt of a metal of group VIB and with a salt of a metal of group VIII,
b) drying and calcination of the material obtained in step a),
c) mixing of the impregnated oxide obtained in step b) with the beta zeolite.
The quantities of the reagents are selected in relation to the composition of the end-catalyst.
The impregnations of step a) are carried out with any traditional method, the salts of metals of groups VIB and VIII are in aqueous solution. When separate aqueous solutions for the metal of group VIB and for the metal of group VIII, are used, a drying and calcination step can be inserted between the two impregnations. Before step c) the impregnated oxide can be ground and sieved into particles of  less than 0.2 mm and then, in step c), mixed with the zeolite by physical mixing or dispersing the particles in an organic solvent of the cyclohexane or cyclohexanol type. The solvent is vaporized and the particles of catalyst dried and calcined. The mixing of step c) can also be carried out by mixing and homogenizing a solid mixture comprising the impregnated oxide (with particle dimensions of  less than 0.2 mm), the zeolite, a ligand and, optionally, combustible organic polymers. The mixture thus obtained can be mixed with a peptizing acid solution, extruded, dried and calcined with any traditional method. Alternatively, the paste can be pelletized, dried and calcined with any traditional method.
The catalysts used in the process of the present invention can be used as such or, preferably, extruded according to the known techniques, for example using a peptizing agent, such as a solution of acetic acid, and optionally a ligand of the pseudobohemite type, added to the catalyst to form a paste which can be extruded. In particular, when the catalysts are prepared by sol-gel, the addition of the ligand is not necessary during the extrusion process.
The materials of the present invention can be used as catalysts for the hydrotreating of hydrocarbon mixtures and more specifically for the upgrading of hydrocarbon mixtures which boil within the naphtha range.
A further object of the present invention therefore relates to the hydrotreating of hydrocarbon mixtures characterized by the use of a catalytic composition which comprises a beta zeolite, a metal of group VIII, a metal of group VIB, and optionally one or more oxides as carrier.
In accordance with this, a particularly preferred aspect of the present invention relates to the hydrodesulfuration of hydrocarbon mixture having boiling ranges within the range of about 35xc2x0 to about 250xc2x0 C., containing olefins and at least 150 ppm of sulfur, with the contemporaneous skeleton isomerization of these olefins, which comprises putting these mixtures in contact, in the presence of hydrogen, with a catalytic composition which comprises a beta zeolite, a metal of group VIII, a metal of group VIB, and optionally one or more oxides as carrier. When the catalytic composition containing the beta zeolite, a metal of group VIB and a metal of group VIII, is used, the process of the present invention is carried out at a temperature ranging from 220 to 360xc2x0 C., preferably between 300 and 350xc2x0 C., at a pressure ranging from 5 to 20 kg/cm2, at a WHSV ranging from 1 to 10 hoursxe2x88x921. The quantity of hydrogen is between 100 and 500 times the quantity of hydrocarbons present (Nl/l).
When the catalytic composition also contains one or more oxides as carrier, the hydrodesulfuration process and contemporaneous skeleton isomerization of the olefins present is carried out at a temperature ranging from 220 to 320xc2x0 C., preferably between 250 and 300xc2x0 C., at a pressure ranging from 5 to 20 kg/cm , and a WHSV between 1 and 10 hoursxe2x88x921. The quantity of hydrogen is between 100 and 500 times the quantity of hydrocarbons present (Nl/l).
The hydrocarbon mixture which can be desulfurated according to the present invention contains more than 150 ppm of sulfur. For example hydrocarbon mixtures with a sulfur content of more than 600 ppm, or even higher than 10,000 ppm can be subjected to hydrodesulfuration.
The hydrocarbon mixtures which are preferably subjected to hydrodesulfuration boil within the range of C5 to about 220xc2x0 C., C5 referring to the boiling point of a mixture of hydrocarbons with five carbon atoms.